It is known to measure the composition and/or flow rate of a fluid by passing the fluid through a radio frequency (RF) electromagnetic field and measuring changes in properties of the electromagnetic field. Conventional multiphase meters which are based on this principle typically comprise a base pipe defining a fluid flow path internally thereof and a concentric cylindrical metallic cavity member arranged around the base pipe. The cavity member provides confinement for an electromagnetic field which extends through the base pipe into the fluid flow path. The base pipe is generally transparent to the electromagnetic field and the cavity member is generally metallic. The cavity member generally supports one or more electromagnetic modes. The electromagnetic modes supported by the cavity member depend not only on the geometry of the cavity, but also on the cavity contents including any fluid present in the fluid flow path. Such known multiphase meters are generally used to measure a frequency response of the electromagnetic field. The measured frequency response may include one or more resonant features associated with the one or more electromagnetic modes. The composition and/or flow rate of any fluid present in the fluid flow path may be extracted from characteristics of the resonant features.
The transparent base pipe is typically configured for connection into a metal pipeline. The cross-sectional geometry of the base pipe is generally configured to match the cross-sectional of the metal pipeline. The inner diameter of the cylindrical cavity member is generally selected to be greater than the outer diameter of the metal pipeline. This may ensure that the fundamental frequency of the electromagnetic field within the cavity defined by the cavity member is less than the minimum frequency that can be propagated in the pipeline so as to minimise the loss of electromagnetic energy from the cavity along the pipeline. In known multiphase meters the annular region defined between the outer surface of the base pipe and the inner surface of the cavity member is generally filled with air or water. Examples of such conventional multiphase meters are described in S. Al-Hajeri, S. R. Wylie, R. A. Stuart and A. I. Al-Shamma'a, “An electromagnetic cavity sensor for multiphase measurement in: the oil and gas industry”, Journal of Physics: Conference Series 76 (2007) 012007; in S. Al-Hajeri, S. R. Wylie, A. Shaw and A. I. Al-Shamma'a “Real time EM waves monitoring system for oil industry three phase flow measurement”, Journal of Physics: Conference Series 178 (2009) 012030; in S. R. Wylie, A. I. Al-Shamma'a, A. Shaw and S. Al-Hajeri, “Electromagnetic cavity sensors for multiphase measurement”, Exploration and Production Oil and Gas Review, Volume 9, Issue 1; and in Finnish patent document no. FI834892.
The Applicant's co-pending UK patent application no. 1218956.9 discloses a fluid sensor comprising a core including a base member such as a base pipe which defines a fluid flow path and a filler member such as a polyether ether ketone (PEEK) filler member located externally of the base member. The fluid sensor further comprises a cavity member for confining an electromagnetic field. The cavity member is located externally of the core. The cavity member may be formed on an outer surface of the core.
Known multiphase meters generally use one of two different antenna configurations for coupling energy between an electrical source and an electromagnetic field within the cavity and/or for coupling energy between the electromagnetic field and an electrical receiver: a monopole antenna including a straight conductor which protrudes into the cavity for coupling with the electric field, or a loop antenna including a electrically conductive loop that protrudes into the cavity and which is connected back to ground for coupling with the magnetic field. To provide acceptable performance, conventional monopole and loop antennas generally protrude radially inwardly from the cavity member by several cm's. As such, conventional monopole and loop antennas generally extend into the annular region which exists in conventional multiphase meters between the outer surface of a base pipe and the inner surface of a cavity member as already described above. A passage is generally formed into or through the core so as to accommodate a monopole antenna or a loop antenna. A potable compound, resin or the like is then used to fill or seal any gaps between the monopole or loop antenna and the passage for environmental integrity and/or electrical insulation. However, the use of conventional monopole and loop antennas in this way may compromise structural strength, environmental integrity and/or electrical insulation. This may be a problem when the multiphase meter is to be used in a hostile environment such as a subsea or a downhole environment.